LOSS OF WATER FROM PLANTS L\ LIQUID FORM 169 



of leaves is adequate to account for the dissipation of all absorbed radiant 

 energy. 



Actually, instead of benefiting plants, transpiration may often be detri- 

 mental. Under conditions of deficient soil water or of high transpiration 

 rates even when the soil water supply is adequate, this process results in a 

 diminution in the water content of a plant and in the turgidity of its cells. 

 Prolonged drought conditions ultimately result in a severe desiccation with 

 the consequent death of all except the most drought resistant species. When 

 the diminution in water content is less severe, a train of other effects such 

 as a decrease in the turgidity of the cells, stomatal closure and reduction in 

 rate or cessation of photosynthesis are induced, all of which have the end 

 result of checking the growth of the plant. It is probably true that lack of 

 water is more often the limiting factor in plant growth than any other single 

 factor. Furthermore deficiency of water is probably responsible for the death 

 of more plants under natural or even cultural conditions than any other 

 single cause. 



The fundamental effects of transpiration upon the plant are not to be 

 sought in any hypothetical "advantages" of the process to the plant, but in 

 its very real and readily ascertainable influences upon the water relations of 

 plant cells and tissues, and through these its effects upon other plant processes. 

 In spite of the fact that transpiration may be regarded in a sense as an in- 

 cidental phenomenon, its indirect influence upon the metabolic processes of 

 plants is a profound one. It is this fact, more than any other, which justifies 

 intensive and critical studies of this process. 



The Loss of Water from Plants in Liquid Form. — If a pot of young 

 oat plants be copiously watered and then enclosed in a bell jar, in a relatively 

 short time a slow exudation of water begins at the tip of each leaf. The 

 drops which form at the leaf apices gradually enlarge and eventually may 

 run down the side of the leaf or fall off. This process of the escape of liquid 

 water from uninjured plants is called guttation (Fig. 40). It is of very 

 general occurrence in plants, having been recorded in plants of more than 

 300 genera although there are many species in which it has not been observed. 

 Guttation occurs most frequently and abundantly under conditions which 

 favor rapid absorption of water by the roots, but which result in a reduced 

 rate of transpiration. In most temperate regions such conditions occur most 

 frequently during the late spring when there is often an alternation between 

 relatively cool nights and relatively warm days. Guttation is frequently 

 observed at that season, usually taking place at night or during the early 

 hours of the morning. The drops of guttation water which form at the tips 



